Whole blood is rarely administered to patients. Rather, patients needing red blood cells are given packed red cells (PRC), patients needing platelets are given platelet concentrate (PC), and patients needing plasma are given plasma. For this reason, the separation of blood into components has substantial therapeutic and monetary value.
The separation of a single unit of donated whole blood into its components is typically accomplished by use of differential sedimentation using centrifugation, as is well known to those skilled in the art. A typical procedure used in the United States utilizes a series of steps to separate donated blood into three components, each component having substantial therapeutic and monetary value. The procedure typically utilizes a blood collection bag which is integrally attached via flexible tubing to at least one, and preferably two or more, satellite bags. Using centrifugation, whole blood may be separated by differential sedimentation into such valuable blood components as plasma, packed red cells (PRC), platelet-rich plasma (PRP), and platelet concentrate (PC).
A typical blood processing procedure may include the following:
(1) The donated whole blood is collected from the donor's vein directly into the blood collection bag which contains a nutrient and anti-coagulant fluid.
(2) The blood collection bag, together with its satellite bags, are placed in a centrifuge bucket. The centrifuge bucket is then placed in the centrifuge. The centrifuge bucket, blood collection bag, and satellite bags are centrifuged together at a slow speed ("soft-spin" centrifugation). Red cells are heavier than other components of the blood. The centrifugal force of the centrifuge concentrates red cells as packed red cells (PRC) in a lower portion, i.e., the sediment layer, of the blood collection bag. The PRC fluid is bright red in color. The volume of the PRC fluid varies considerably depending on the number of red cells contained in the drawn whole blood (typically 37-54 percent by volume). A suspension of platelets in clear plasma, known as platelet-rich plasma (PRP) remains in the upper portion, i.e., the supernatant layer of the blood collection bag. The PRP fluid is light yellow in color. The interface between the supernatant PRP layer and the sediment PRC layer is known as the buffy coat interface. The location of the buffy coat interface can be determined by a visual inspection of the blood collection bag. This interface is seen as the point where the light yellow PRP fluid merges with the bright red PRC fluid.
(3) After centrifugation, the blood collection bag is transferred, with care not to disturb the buffy coat interface, into a device known as an expressor. A typical expressor is formed by a front and a back plate which are hinged together at their lower ends and spring biased toward each other. The blood collection bag is placed between the plates. The spring is released, and the plates are forced together, compressing the blood collection bag. A valve or clamp in the flexible tubing is opened, and the fluid is squeezed out of the blood collection bag by the expressor. The supernatant PRP layer is positioned at the top of the blood collection bag, and is therefore the first fluid to flow out of the collection bag. The supernatant PRP fluid flows into a first satellite bag.
An alternative means of expressing the fluid is a pressure cuff. The pressure cuff is constructed similar to a blood pressure cuff commonly used to determine a patient's blood pressure. The pressure cuff is wrapped around the blood collection bag. As the pressure cuff is inflated, the pressure cuff expands and bears against the collection bag, expressing the blood from the collection bag. Gravity may also be used to express the fluid from the blood collection bag.
As the PRP flows out of the blood collection bag, the buffy coat interface with the PRC rises. The operator closely observes the position of the buffy coat interface as it rises and clamps off the connecting tube when in his judgment as much PRP has been transferred as is possible, without allowing red cells to enter the first satellite bag. This is a labor intensive and time consuming operation during which the operator must visually monitor the bag and judiciously and arbitrarily ascertain when to shut-off the connecting tube.
The blood collection bag, now containing only PRC, may be detached and stored until required for transfusion into a patient. Alternatively, a valve or seal in the tubing may be opened so that the PRC may be transferred to a second satellite bag by means of the expressor.
(4) The PRP-containing satellite bag and another satellite bag are then removed from the expressor and centrifuged at an elevated G force (high speed or "hard-spin" centrifugation) with the time and speed adjusted so as to concentrate the platelets into the lower portion of the PRP bag. When centrifugation is complete, the PRP bag contains sediment platelet layer (light yellow in color) in its lower portion and supernatant clear plasma layer in its upper portion.
(5) The PRP bag is then placed in the expressor, and most of the clear plasma is expressed into a satellite bag, leaving the PRP bag containing only the sediment platelet layer and a small amount of residual plasma. In a subsequent step, the sedimented platelet fluid may be processed to make platelet concentrate (PC). The PRP bag, now containing a PC product, is then detached and stored until needed for a transfusion of platelets. For use with adult patients, the platelets from 4-8 donors may be pooled into a single platelet transfusion.
(6) The plasma in the satellite bag may itself be transfused into a patient, or the plasma may be separated by complex processes into a variety of valuable products.
In the above described procedures it is important to determine where the supernatant PRP fraction ends and the sediment PRC fraction begins. In separating the PRC and PRP fractions (e.g., step 3 above), blood bank personnel have attempted to ensure that the entire PRP fraction is recovered. This has often proved to be counterproductive since the PRP fraction may become contaminated by red cells from the buffy coat interface or the PRC fraction--giving a pink or red color to the normally light yellow PC. The presence of red cells in PC is so highly undesirable that pink or red PC is frequently discarded, or subjected to re-centrifugation, both of which increase operating costs and are labor intensive. As a result, blood bank personnel must err on the side of caution carefully observing the buffy coat interface and by stopping the flow of PRP before it has been fully expressed. Although the PRP is uncontaminated, valuable unexpressed plasma may be wasted.
Conventional expressors used in the above-described procedures have many drawbacks. For example, they apply an uneven pressure to the blood collection bag and may create wrinkles and folds in the bags. Blood products become trapped in these wrinkles and folds, preventing 100% of the fluid from being expressed. Uneven pressure also tends to disturb the buffy coat interface and reduce the amount of PRP which can be reliably collected. Additionally, because the bag may be distorted and because the structure of convention expressors can hinder observation of the container, it is very difficult to watch the buffy coat interface level to determine when to terminate the flow of the PRP fraction.
Further, in some applications, it is desirable to draw fluid into a container. For example, after a supernatant PRP layer has been expressed from a collection bag, it may be desirable to draw an additive solution into the collection bag and add it to the remaining PRC layer. However, conventional expressors are merely capable of squeezing a container. So while they can force fluid out of the container, they are unable to draw fluid into the container.